1. Field of the Invention
This invention relates to bearings. More particularly, this invention relates to bearings which are interference fit into receiving structural elements which support relatively high reversing loads.
2. Description of the Prior Art
The simplest way to transfer a load from one structural element to another is to rigidly fasten the two together with some sort of fastening means such as a rivet or a bolt or to weld or glue the two elements together. However, it is often necessary that provision be made for some amount of movement between the two structural elements which largely precludes these simple joining measures. To meet this need, various different types of bearings have been developed, depending upon the particular application needed and the transference of the load from one element to another. Since this invention was initially developed in the context of that class of bearings known as spherical bearings, the prior art will be discussed in this rather limited area. However, it should be realized and, indeed, immediately apparent that the application of this invention is not limited solely to spherical bearings but to essentially any bearing assembly susceptible to the interference fit concepts presented herein.
There are several types of spherical bearings in use today. Two types are the captive ball and the two piece ball and outer race type of spherical bearings. In applications in the aircraft industry, these bearings are commonly used to transfer a reversing load from a shaft such as a bolt to an attachment point, commonly a relatively thick planar section of high strength aluminum through which the shaft passes. With both types of spherical bearings, there is an inner ball means through which the shaft passes and an outer race means which slides into the aluminum structural support means. The relative motion between the shaft and the aluminum support element is taken up by the spherical bearing sliding surface found between the inner ball means and the outer race means. Turning to the captive ball type of spherical bearing, this type is formed by a process in which a straight wall liner bushing is centered on the ball and then swaged down to the shape of the ball. During this formation process, it is impossible to achieve 100% sliding contact with the ball since elastic spring back of the liner bushing and an inherent arching effect produced thereby combine to leave considerably less than a 50% contact area in the spherical contacting surfaces of the bearing. This assembly is then lightly roll released or impact hammered to stretch the outer race to provide a closer sliding fit with only about 50% contact area. Nevertheless, the arched gap remains. An unfortunate side effect of the formation of the outer race is that the bearing sliding surface of the outer race is relatively rough due to the swaging and limited roll releasing. This type of bearing is always installed with a net to a very low press fit to prevent binding and freezing the movement of the spherical bearing. This low press fit and the inherent voids between the ball and the outer race, commonly at least 50%, cause fretting, low fatigue life and significant axial movement of the bearing measured along the axis of the shaft within the structural housing. Roll staking of the bearing does not restrict this movement under high reversing loads, and high impact loads on the bearing increase the radial clearances, which in turn aggravate fretting and stress corrosion to accellerate bearing failure. The second type of spherical bearing in use today is the two piece ball and outer race spherical bearing. In this type of spherical bearing, the ball is in two pieces which are slip fit through and rotated into the outer race. These spherical ball bearings are precision ground and lapped to provide a 100% sliding contact fit between the ball and the outer race. This largely avoids the arching problems inherent in the above discussed bearing; however, these bearings are 50% more expensive, and the two piece ball presents some application problems. These bearings must also be installed with a net to low press fit to prevent freezing up movement of the bearing. With a 100% spherical bearing ball contact area, this type of bearing is much easier to freeze up than is the swaged captive ball type of spherical bearing. As with the captive ball type spherical bearing, the two piece ball and outer race spherical bearing, due to its low press fit, allows significant axial and inline movement, which in turn leads to stress corrosion and fretting and resulting failure of the bearing.
Two more types of spherical bearings in current use are the Messerschmidt or "window slot" spherical bearing and a variation on the swaged captive ball bearing discussed above. The Messerschmidt bearing has a significant portion of the outer race cut out to allow for the insertion of the inner ball. This cutout seriously weakens the bearing. The swaged captive ball variation starts with an outer race which is machined to fit one half of the inner ball with the other half being a straight wall bushing into which the inner ball is inserted. Once the ball is inserted, the straight wall half is swaged down onto the inner ball to capture it. As with the previous two types of spherical bearings, these last two types are installed with net sliding fits or low press fits, and, as above, the inherent tolerances necessary for these fits allow for the cyclic motion under load which causes relatively rapid failure in all the prior art spherical bearings.
None of the prior art spherical bearings lend themselves to high press interference fit applications. Such applications would automatically produce binding and freezing of the spherical bearings.